The invention relates to a process and apparatus used to test the porosity of coated objects and, in particular, of enamelled, electrically conductive strip and profiled materials or objects or of hollow bodies such as beverage cans.
The porosity of such coatings when applied to hollow bodies can be measured by filling the hollow body with an electrolytic fluid, applying to the hollow body one pole of a test voltage and applying the other pole to an electrode immersed in the electrolytic liquid and measuring the flow of current. The test voltage will as a rule be 6.3 V. If the coating exhibits no porosity, no current flow will be indicated. Depending on quality requirements, a certain degree of porosity may be tolerated and this can be defined by specifying a permissible maximum value for the flow of current. If the permissible threshold value is exceeded the hollow body will have to be marked and/or rejected and may possibly be recoated.
The method described above for the testing of the porosity of beverage cans is time-consuming and costly since it is necessary to fill the beverage cans individually with electrolytic fluid, make the measurement and then to empty, rinse and dry the beverage cans. For this reason the test process known in the art is performed only on a spot-check basis following the coating of the beverage cans.
The procedure described above can be applied only in testing the porosity of the internal coating of beverage cans and similar hollow bodies, but not in testing coated strip and profiled material.